The present invention relates a color-separating and -recombining optical system having several prism assemblies each made up of prisms bonded to each other with an optical adhesive and being applicable to a projection display having spatial light modulators.
Color projection displays operate as follows: White light is separated into three primary colors R (Red), G (Green) and B (blue). The separated color components are guided to the corresponding spatial light modulators (abbreviated to SLM hereinafter) for optical modulation in accordance with a video signal. The modulated color components are recombined and projected onto a screen, thus a color image being displayed thereon.
Color projection displays are classified into three types in accordance with SLMs to be used, such as, a type with transparent SLMs, another with reflective SLMs, and still another with a DMD (Digital Mirror Device).
Compact transparent SLMs and DMDs are available for their relatively simple optical architecture but have difficulty in resolution.
On the contrary, reflective SLMs exhibit high resolution but pose a problem in compactness due to complex optical system using this type of SLMs. Particularly, projection displays equipped with reflective SLMs require polarization beam splitters (abbreviated to PBS hereinafter) for splitting light beams incident into the SLMs and reflected light beams that have been modulated by the SLMs. In detail, each reflective SLM requires two or more of PBSs for high contrast, thus resulting in complex optical architecture for reflective projection displays.
A compact color-separating and -recombining optical system with no such problems on optical architecture of reflective SLMs is disclosed, for example, in Japanese Unexamined Patent Publication No. 2002-228809.
Nonetheless, projection displays equipped with such a compact color-separating and -recombining optical system suffer low contrast at the corners of a black image on screen due to birefringence when a high-intensity discharge lamp of 100 W or more is used. The birefringence occurs when a known optical glass BK7 is used for the transparent material of PBSs.
In order to solve such a problem, Japanese-Unexamined Patent Publication No. 9-54213 discloses that a specific transparent material of small opto-elastic constant for PBSs causes small birefringence. It is taught that the specific transparent material of small opto-elastic constant is effective when used for a main PBS that splits a light beam incident into a reflective SML and another light beam modulated and reflected by the reflective SLM.
The refractive index Nd of optical adhesives is relatively low, for example, in the range from 1.45 to 1.6 for epoxy and ultraviolet-cured optical adhesives, mostly about 1.5.
A well-known optical glass, borosilicate glass, such as BK7, exhibits a refractive index of about 1.5.
In contrast, glass materials having small photoelastic constants are usually heavy materials containing a large amount of lead, such as optical flint, exhibiting high refractive indexes of about 1.85.
The difference in refractive index Nd between a borosilicate glass, such as BK7, and a bonding layer of optical adhesive is thus small in a prism assembly, and hence the existence of the bonding layer can be ignored for a light beam passing through the prism assembly.
On the contrary, the difference in refractive index Nd between a glass of a small photoelastic constant and a bonding layer of optical adhesive is large in a prism assembly. This large difference in refractive index causes wavefront aberration or error due to refraction of a light beam on the interface of the bonding layer, thus resulting in low resolution.